1. Field of the Invention
The present invention relates to a visual axis detection apparatus, and especially to a visual axis detection apparatus which detects an axis in an observation point direction of a viewer (photographer) or a so-called visual axis when the viewer observes an observation plane (imaging plate) on which an object image is formed by a photographing system in an optical system such as a camera, by utilizing a reflected image (eyeball image) formed when an eyeball of the viewer is illuminated with an infrared ray.
2. Related Background Art
Various visual axis so as detection apparatuses for detecting the visual axis to detect a position on a view plane which the viewer (examined person) views have been proposed.
For example, in Japanese Laid-Open Patent Application No. 2-264632, an infrared light beam from a light source is projected to an anterior part of the eye in an eye to be examined and an axis of vision (observation point) is determined by utilizing a cornea reflected image on the basis of light reflected from a cornea and a focus-imaging point on a pupil.
In a camera disclosed in Japanese Laid-Open Patent Application No. 61-61135, a the direction of metering by a focus detection apparatus is mechanically controlled on the basis of an output signal from a visual axis detection means to adjust a focal point state of a photographing system.
FIG. 5 is a schematic view of a visual axis detection apparatus proposed in Japanese Laid-Open Patent Application No. 2-264632, FIG. 6 is a graph of an output signal from one line of an image sensor of FIG. 5, and FIG. 7 is a perspective view of a portion of a finder system when the visual axis detection apparatus of FIG. 5 is applied to a single eye reflex camera.
Numeral 101 denotes an eyeball of an examined person (observer), numeral 1 denotes a cornea of the eyeball of the examined person, numeral 2 denotes a sclera, and numeral 3 denotes an iris. O' denotes a center of rotation of the eyeball 101, O denotes a center of curvature of the cornea 1, a and b denote ends of the iris 3, and e and f denote positions where cornea reflected images are formed owing to light sources 4a and 4b to be described hereinafter. Numeral 4a and 4b denote light sources which may be light emitting diodes or the like for emitting infrared rays which are unpleasant for the examined person. The light source 4a (4b) is arranged closer to a projection lens 6a (6b) than to a focal plane of the projection lens 6a (6b). The projection lenses 6a and 6b are applied for widely illuminating the cornea 1 defining a light beam from the light sources 4a and 4b as diverged light beam.
The light source 4a lies on an optical axis of the projection lens 6a and the light source 4b lies on an optical axis of the projection lens 6b, and they are arranged symmetrically along a z-axis direction with respect to an optical axis aX.sub.1.
Numeral 7 denotes a light receiving lens which forms the cornea reflected images e and f formed near the cornea 1 and the ends a and b of the iris 3 on an image sensor plane 9. Numeral 10 denotes an arithmetic means which calculates the visual axis of the examined eye by using the output signal from the image sensor 9. aX.sub.1 denotes an optical axis of the light receiving lens 7 and it matches the X axis in FIG. 5. aX.sub.2 denotes an optical axis of the eyeball which makes an angle .theta. with respect to the X axis.
In this example, the infrared ray emitted from the light source 4a (4b) passes through the projection lens 6a (6b) and thereafter widely illuminates the cornea 1 of the eyeball 101 in a diverging state. The infrared ray which passes through the cornea 1 illuminates the iris 3.
The cornea reflected images e and f based on the light beam reflected by the surface of the cornea 1 of the infrared rays for illuminating the eyeball are reformed at points e' and f' on the image sensor 9 through the light receiving lens 7. In FIGS. 5 and 6, e' and f' denote projection images of the cornea reflected image (virtual images) e and f formed by a set of light sources 4a and 4b. The centers of the projection images e' and f' substantially match to the projection point on the image sensor 9 of the cornea reflected image formed when the illumination means is arranged on the optical axis aX.sub.1.
The infrared ray which is diffusion-reflected by the surface of the iris 3 is directed to the image sensor 9 through the light receiving lens 7 to form the iris image.
On the other hand, the infrared ray transmitted through the pupil of the eyeball to illuminate a retina has the wavelength of the infrared range and the illuminated area is an area of a low view cell density which is apart from a center area, so that the examined person cannot recognize the light sources 4a and 4b.
The ordinate in FIG. 6 represents an output I along the z-axis direction of the image sensor 9. Since most of the infrared rays transmitted through the pupil are not reflected back, there is no difference in the output at the boundary between the pupil and the iris 3. As a result, the iris images a' and b' at the ends of the iris can be detected.
When an area sensor having a two-dimensional photo-sensor array is used as the image sensor 9 of FIG. 6, two-dimensional light distribution information of the reflected image (eyeball image) is obtained from the front eye as shown in FIG. 8.
In FIG. 8, numeral 141 denotes a light receiving area of the image sensors, E' and F' denote cornea reflected images of the light sources 4a and. 4b, A' denotes a boundary between the iris and the pupil, and G' denotes a boundary between the sclera 2 and the cornea 1. Since the reflectivities of the sclera 1 and the iris 3 are not substantially different from each other in the infrared range, the boundary G' can not be clearly discriminated by a naked eye. J' denotes an image of a lower eyelid, K' denotes an image of an upper eyelid and L' denotes an image of eyelashes.
In order to detect the direction of the visual axis from the eyeball image of the front part of the eye, it has been known to calculate the relative relation between the cornea reflected images E' and F' (or an intermediate image of E' and F') and the position of the center of pupil. Various methods for determining the center of pupil have been known. For example, an output of one particular line of the image sensor is sampled to calculate a center point of the pupil edge positions a' and b' of FIG. 6. Alternatively, the output information of the area sensor may be used to sample a number of pupil edge points and thereafter determine the center point by a least square approximation.
Optional equipment having a finder system such as a still camera or a video camera is frequently used in out-of-door conditions. When such optical equipment is used out-of-doors, the eyeball of the photographer is illuminated by an external ray. Thus, an image forming light beam received by the image sensor 9 includes not only the image of the front part of the eye illuminated with the light sources 4a and 4b but also a complex image affected by the disturbance by the external ray.
The external ray causing the most problems is a direct light incident on the front part of the eye from the sun. The energy of sunlight is very strong and includes a large amount same spectrum components as those emitted by the light sources 4a and 4b. Accordingly, it is difficult to fully eliminate the external ray by spectrum means such as a visible ray cut filter.
When the front part of the eye is illuminated by sunlight, a variety of disturbances are generated in the image and the external ray component is stronger than the infrared component. As a result, a pattern (eyeball image) cannot be substantially discriminated. When the external ray exists, the brightness in the pupil which should be at a lowest brightness level of luminescence (between a' and b' in FIG. 6) becomes higher or declines so that the detection of the pupil edges and hence the position of the center of the pupil cannot be correctly determined.
When the neighborhood of the boundary of the sclera and the iris is strongly illuminated, an obscure edge, which inherently seems unclear, rises to the surface or declines therein, so that the pupil edges are misdetected. When the eyelashes grow downward, they are illuminated by the external ray, so that they may be misdetected as the pupil edge. Since the eyelashes extend out of the face in contrast to the eyeball, they are easily subject to the illumination by the external ray.
Such a misdetection occurs not only for the pupil edge but also for the cornea reflected images e and f of the light sources 4a and 4b. When the ends of the eyelashes are directly illuminated by the sunlight, they become strong brilliant points, which are misdetected as the cornea reflected images. When eyeglasses are put, on dust deposited on the eyeglasses may be highlighted.
Besides sunlight, a downwardly directed light having high luminescence and various artificial light sources may form also an external e ray. When eyeglasses are put, on a distance between the eyepiece portion in the finder system and the eyeball generally becomes separated so that the external ray easily enters into the eye. Further, the reflection coming from the lens surfaces of the eyeglasses is adversely affected.
When the visual axis is to be detected by using the image signal from the image sensor, an accumulation-type image sensor is frequently used in view of the sensitivity requirements for the system. As a result, there has been a problem that DC noise elimination by an AC coupling or a period detection system which is usually used in a single sensor cell cannot be used.